Method of fabricating single crystal gallium nitride semiconductor substrate, nitride gallium semiconductor substrate and nitride semiconductor epitaxial substrate

ABSTRACT

A method of fabricating a single crystal gallium nitride substrate the step of cutting an ingot of single crystal gallium nitride along predetermined planes to make one or more single crystal gallium nitride substrates. The ingot of single crystal gallium nitride is grown by vapor phase epitaxy in a direction of a predetermined axis. Each predetermined plane is inclined to the predetermined axis. Each substrate has a mirror polished primary surface. The primary surface has a first area and a second area. The first area is between an edge of the substrate and a line 3 millimeter away from the edge. The first area surrounds the second area. An axis perpendicular to the primary surface forms an off-angle with c-axis of the substrate. The off-angle takes a minimum value at a first position in the first area of the primary surface.

RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.11/498,155, filed on Aug. 3, 2006, the disclosure of which Applicationis incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of fabricating a singlecrystal gallium nitride substrate, a gallium nitride substrate and anitride semiconductor epitaxial substrate.

2. Related Background Art

Publication 1 (Japanese Patent Application Laid Open No. 2001-196632)discloses a nitride based compound semiconductor light emitting device.This nitride based compound semiconductor light emitting device has anactive layer and an acceptor-doped layer of nitride-based compoundsemiconductor on the surface of a GaN substrate, and the surface ofcrystal orientation is inclined in the range of 0.05 degrees to 2degrees with reference to <0001> direction.

Publication 2 (Japanese Patent Application Laid Open No. 2000-223743)discloses a nitride based semiconductor light emitting device. Thisnitride based semiconductor light emitting device has a light generatinglayer of nitride-based semiconductor on the surface of a GaN substrate.In the nitride based semiconductor light emitting device, the surface ofcrystal orientation is inclined in the range of 0.03 degrees to 10degrees.

Publication 3 (Japanese Patent Application Laid Open No. 2000-22212)discloses a GaN wafer. This GaN wafer is formed by polishing. Theoff-angle of the surface of the GaN wafer is within 3 degrees, thevariation of the off-angle is within 4 degrees over the surface. Thewarpage of the GaN wafer is within 200 micrometers.

SUMMARY OF THE INVENTION

Each of the nitride semiconductor light emitting device in Publications1 and 2 discloses has a GaN base. The nitride semiconductor lightemitting devices are fabricated by dividing the wafer into semiconductordies. The size of each semiconductor die is at most about onemillimeter. Nitride semiconductor light emitting devices as shown inPublications 1 and 2 are obtained from part of the GaN wafer because thesurfaces of available GaN wafers at present have off-angle variationsnot less than desired off-angle range.

What is needed is to fabricate nitride semiconductor devices havingdesired device characteristics on a wider part of the surface of one GaNwafer.

It is an object of the present invention to provide a single crystal GaNsubstrate which can provide nitride semiconductor devices having desireddevice characteristics on a greater part of the surface thereof. It isanother object to provide a method of fabricating a single crystal GaNsubstrate. It is still another object to provide a nitride semiconductorepitaxial substrate using this GaN substrate.

One aspect of the present invention is a method of fabricating a singlecrystal gallium nitride substrate. The method comprises the step of:cutting an ingot of single crystal gallium nitride along predeterminedplanes to make one or more single crystal gallium nitride substrates.

The ingot of single crystal gallium nitride is grown by vapor phaseepitaxy in a direction of a predetermined axis. Each predetermined planeis inclined to the predetermined axis. Each substrate has a mirrorpolished primary surface. The primary surface has a first area and asecond area. The first area is between an edge of the substrate and aline 3 millimeter away from the edge. The first area surrounds thesecond area. An axis perpendicular to the primary surface forms anoff-angle with c-axis of the substrate. The off-angle takes a minimumvalue at a first position in the first area of the primary surface.

Another aspect of the present invention is a method of fabricating asingle crystal gallium nitride substrate. The method comprises the stepof slicing an ingot of single crystal gallium nitride alongpredetermined planes to prepare one or more single crystal galliumnitride slices. Each predetermined plane is inclined to a predeterminedaxis. The ingot of single crystal gallium nitride is grown by vaporphase epitaxy, and each single crystal gallium nitride slice has a slicesurface.

The method further comprises the step of performing at least one processof polishing and grinding of the slice surface to form one or moresingle crystal gallium nitride substrates. Each substrate has a primarysurface. The primary surface has a first and second areas. The firstarea is between an edge of the substrate and a line 3 millimeters awayfrom the edge. The first area surrounds the second area. An axisperpendicular to the primary surface forms an off-angle with c-axis ofthe substrate. The off-angle takes a minimum value at a first positionin the first area of the primary surface.

In the method according to the present invention, an axis perpendicularto the primary surface forms an angle greater than zero with c-axis ofthe substrate.

In the method according to the present invention, the off angle takes amaximum value at a second position on the primary surface, and the offangle makes substantially monotonic change on a segment from the firstposition to the second position.

In the method according to the present invention, the off angle takes anonzero constant value on a curve on the primary surface and the curveterminates at the edge of the primary surface.

Still another aspect of the present invention is a gallium nitridesubstrate of single crystal gallium. The gallium nitride substratecomprises a primary surface. The primary surface has a first area and asecond area. An off-angle formed by c-axis of the gallium nitridesubstrate with a axis perpendicular to the primary surface is greaterthan zero over the first and second areas.

Yet another aspect of the present invention is a gallium nitridesubstrate of single crystal gallium. The gallium nitride substratecomprises a primary surface. The primary surface has a first area and asecond area. The first area is located between an edge of the primarysurface and a line 3 millimeter apart from the edge. The off-angleformed by c-axis of the gallium nitride substrate with a axisperpendicular to the primary surface is greater than zero over thesecond area.

In the gallium nitride substrate according to the present invention, theoff-angle takes a minimum in the first area.

In the gallium nitride substrate according to the present invention, theoff-angle takes a value greater than zero on a curve on the primarysurface, and the curve terminates on an edge of the primary surface.

In the gallium nitride substrate according to the present invention, amaximum distance between one position and another position on the edgeof the primary surface is equal to or greater than 10 millimeters.

In the gallium nitride substrate according to the present invention, anarea of the primary surface is equal to or greater than an area of2-inch diameter circle.

In the gallium nitride substrate according to the present invention, theoff-angle is equal to or greater than 0.15 degrees all over the secondarea.

In the gallium nitride substrate according to the present invention, theoff-angle is equal to or greater than 0.3 degrees all over the secondarea.

In the gallium nitride substrate according to the present invention, theoff-angle is less than 2 degrees all over the second area.

In the gallium nitride substrate according to the present invention, theoff-angle is equal to or less than 0.7 degrees all over the second area.

An epitaxial substrate according to the present invention comprises: agallium nitride substrate according to any one of the gallium nitridesubstrate; and one or more III-group nitride semiconductor film providedon the gallium nitride substrate.

An epitaxial substrate according to the present invention comprises: agallium nitride substrate according to any one of the gallium nitridesubstrate; a first conductive type Al_(X1)In_(Y1)Ga_(1-X1-Y1)N (0≦X1≦1,0≦Y1≦1, 0≦X1+Y1≦1) provided on the gallium nitride substrate; an activeregion including an Al_(X2)In_(Y2)Ga_(1-X2-Y2) (0≦X2≦1, 0≦Y2≦1,0≦X2+Y2≦1) provided on the gallium nitride substrate; and a secondconductive type Al_(X3)In_(Y3)Ga_(1-X3-Y3)N (0≦X3≦1, 0≦Y3≦1, 0≦X3+Y3≦1)provided on the gallium nitride substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and other objects, features, and advantages of thepresent invention will be understood easily from the following detaileddescription of the preferred embodiments of the present invention withreference to the accompanying drawings:

FIG. 1 is a view explaining a method of fabricating a single crystalgallium nitride substrate;

FIG. 2 is a view showing an ingot of single crystal gallium nitridesemiconductor;

FIG. 3 is a view showing a single crystal gallium nitride substrate;

FIG. 4 is a graph showing the relationship between the surface roughnessof a GaN film on a GaN wafer and off-angle of the GaN wafer;

FIG. 5 is a view showing the distribution of off-angle over the surfaceof the GaN single crystal substrate shown in part (B) of FIG. 3;

FIG. 6 is a view showing the distribution of off-angle over the surfaceof the GaN single crystal substrate shown in part (D) of FIG. 3;

FIG. 7 is a view showing the distribution of off-angle over the primarysurface of another GaN single crystal substrate;

FIG. 8 is a graph showing relationship between off-angle and thedistance from the reference point;

FIG. 9 is a view showing process steps for the method of fabricating agallium nitride-based semiconductor device and the method of forming agallium nitride-based epitaxial substrate;

FIG. 10 is a view showing process steps for the method of fabricating agallium nitride-based semiconductor device and the method of forming agallium nitride-based epitaxial substrate;

FIG. 11 is a view showing the distribution of photo-luminescencewavelength of the epitaxial wafer formed by the method in FIG. 2 andshowing the histogram of this distribution; and

FIG. 12 is a view showing the distribution of photo-luminescenceintensity of the epitaxial wafer formed by the method in FIG. 2 andshowing the histogram of this distribution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The teachings of the present invention will readily be understood inview of the following detailed description with reference to theaccompanying drawings illustrated by way of example. Referring to theaccompanying drawings, embodiments of a nitride semiconductor wafer, thenitride semiconductor wafer and a nitride semiconductor epitaxialsubstrate of the present invention will be explained. When possible,parts identical to each other will be referred to with symbols identicalto each other.

Part (A) in FIG. 1 shows a single crystal gallium nitride, part (B)shows a III-V compound semiconductor wafer, and part (C) shows a crosssection, taken along I-I line in part (A), of the single crystal galliumnitride according to the present embodiment.

An ingot of gallium nitride semiconductor single crystal formed by avapor phase deposition method is prepared. Referring to part (A) in FIG.1, a gallium nitride semiconductor single crystal ingot 1 is shown. Thisingot 1 is fabricated as follows. For example, a mask is formed on theIII-V compound semiconductor substrate 3, such as GaAs (111) singlecrystal wafer, as shown in part (B) in FIG. 1. This mask has windowsarranged in [11-2] direction and [-111] direction. A GaN buffer layer isgrown in these windows at a lower temperature. Next, another GaN layeris deposited on the mask and GaN buffer layer at a higher temperatureusing a vapor phase deposition method, such as hydride vapor phaseepitaxial growth (HVPE) method. After this growth, the GaAs wafer iseliminated therefrom to form a GaN single crystal substrate 5. GaAs canbe removed by use of etchant, such as aqua regalis. A thick GaNepitaxial layer having a thickness of at least 10 millimeters is grownon the GaN single crystal semiconductor substrate 5 to form the ingot 1.

In order to form the single crystal ingot 1, crystal is grown by a vaporphase epitaxy in the direction Ax of a predetermined axis. As shown inpart (B) of FIG. 1, the III-V compound semiconductor substrate 3 doesnot have a substantial warpage. The single crystal ingot 1 has a convexor concave warpage depending on its fabrication process condition and/orits thickness. In part (C) of FIG. 1, a GaN surface 1 a is concave, andC-faces ((0001) face) 3 a, 3 b, 3 c in the single crystal GaN ingot 1 isschematically shown.

Parts (A) and (B) of FIG. 2 show cross sections taken along I-I line inPart (A) of FIG. 1. In part of FIG. 2, C-axes, C1, C2 and C3 are shown.The predetermined axis Ax extends on a C-axis. For example, distancesD1, D2 and D3 between the axis C1 and axis C2 are changed to be narrowas a coordinate in the Z-axis direction is increased. These c-faces areconcave or convex.

The GaN substrate is formed in the following method, for example. Thesingle crystal ingot 1 is cut along predetermined planes S1, S2 and S3to form one or more single crystal GaN substrates. These predeterminedplanes are inclined with reference to the axis Ax extending in thelongitudinal direction of the ingot 1.

These predetermined planes S1, S2 and S3 are not perpendicular to anyC-axes of the ingot 1. The predetermined planes S1, S2 and S3 areperpendicular to an axis Ox that does not intersect with the ingot 1.For example, the plane S1 (planes S1 and S2 in the same manner)intersects with a C-axis of the ingot 1 to form an angle thatmonotonically increases as X coordinate of the intersection isincreased.

In part (B) of FIG. 2, C axes C1-C2 and C3 are shown as in part (A) ofFIG. 2. The axes Ax and Ox extend on a certain C axis and the Ox axisextends in the longitudinal direction of the ingot 1.

In an example of part (B) in FIG. 2, one or more GaN single crystalsubstrates are formed by cutting the single crystal ingot 1 alongpredetermined planes T1, T2 and T3.

These predetermined planes T1, T2 and T3 are perpendicular to the axisOx in the ingot 1. For example, the plane T1 (planes T1 and T2 in thesame manner) intersects with a C-axis passing through a position of theingot 1 to form an angle that monotonically increases as the relevantintersection is spaced apart from the axis Ox in the X-axis direction.

Part (A) in FIG. 3 shows the single crystal GaN substrate obtained bythe method in part (A) of FIG. 2. The surface 7 a of the single crystalGaN substrate 7 is mirror finished. The surface 7 a of the singlecrystal GaN substrate 7 has a first area 7 b between the edge of thesubstrate 7 and a line distanced from this edge by 3 millimeters, and asecond area 7 c inside the first area 7 b. The dash line in part (A) ofFIG. 3 indicates the boundary between the first areas 7 b and secondareas 7 c.

Part (B) in FIG. 3 is a cross sectional view, taken along II-II line,showing one substrate of the substrates fabricated by slicing the ingot1 along the planes S1, S2 and S3. Part (C) in FIG. 3 is a crosssectional view showing one substrate of the substrates fabricated byslicing the ingot 1 along the planes T1, T2 and T3. A number of C axesare depicted in solid line in parts (B) and (C) of FIG. 3. In a singlecrystal GaN substrate 9, the off-angle at the middle of the surface ofthe GaN substrate 9 is zero and the off-angle (Ang1, Ang2: Ang1<Ang2) ata position on this surface is increased as the relevant position movesapart from the center. On the contrary, the surface 7 a of the GaNsubstrate 7 does not have an area on which the off-angle is zero. In themethod according to this embodiment, the ingot 1 is sliced along thepredetermined planes to form the single crystal GaN substrate 7, and theoff-angle on the surface 7 a becomes minimum at a first position in thefirst area 7 b (at a position 7 e on the edge in the present example).The surface of a film grown on an area of the substrate on which theoff-angle is close to zero has the surface morphology of six-sidedpyramid, which may prevent the further improvement of semiconductordevices formed thereon. The off-angle is greater than zero all over thesurface, and takes the minimum value at the position 7 e in the firstarea 7 b and this minimum is not zero.

At a position in the first and second areas 7 b and 7 c, the axisperpendicular to the primary surface 7 a form a nonzero angle (Ang3,Ang4: Ang3<Ang4) with the C-axes. The present fabrication methodprovides a single crystal GaN substrate the surface of which does nothave an off-angle of zero. According to this single crystal GaNsubstrate, since the off-angle is greater than zero all over the primarysurface, a semiconductor film having an excellent surface morphology canbe formed on single crystal GaN substrate.

Part (D) of FIG. 4 shows a single crystal GaN substrate fabricated by amodified method thereof. According to this method, the ingot of singlecrystal GaN is grown in the predetermined Ax by a vapor phase epitaxy,and is sliced along the predetermined planes S1, S2 and S3 to form oneor more single crystal GaN slices. For example, a single crystal GaNsubstrate 11 is formed by providing the surface of one of the singlecrystal GaN slices with the processing of grinding and/or polishing.

Provided are the single crystal GaN substrate 11 the off-angle of whichtakes the minimum at the first position in the first area 7 b accordingto the present fabrication method, since the single crystal slices areformed by slicing the ingot along the predetermined planes.

The off-angle takes the minimum at the position 11 e in the first area11 b of the primary surface 11 a. The minimum is equal to zero in thepresent example. In the fabrication of semiconductor devices, the outerregion 11 b of the substrate 11 (outside a line spaced 3 millimetersapart from the substrate edge) is not used for fabricating thesemiconductor devices, and thus this outer region may contain a positionor an area in which the off-angle is zero.

According to this GaN substrate 11, the off-angle is greater than zeroall over the second area 11 c of the primary surface, a semiconductorfilm having an excellent surface morphology in the second area 11 c canbe deposited.

In the substrate 7 (11) in part (B) of FIG. 3 (part (D) of FIG. 3), theoff-angle takes the minimum at the first position 7 e (11 e) on theprimary surface 7 a (11 a) and takes the maximum at the second position7 f (11 f). The off-angle is monotonically changed on the segment thatconnects the first position of the minimum off-angle to the secondposition of the maximum off-angle. According to the substrate 7, 11,since the off-angle monotonically is changed on this segment, the singlecrystal GaN substrate 7, 11 does not have a region in which theoff-angle on the second area 7 c (11 c) is zero.

In the GaN substrate 7, 11, the maximum distance from the first positionto the second position on the edge of the GaN substrate 7, 11 can begreater than 10 millimeters. Further, according to the GaN substrate 7,11, the area of the primary surface thereof is equal to or greater thanthat of the circle of 2-inch diameter.

FIG. 4 is a graph showing the relationship between the surface roughnessof a GaN film on a GaN wafer and the off-angle. The horizontal axisindicates the off-angle of the GaN substrate and the longitudinal axisindicates the surface roughness (average root mean square surfaceroughness: Rms) of the GaN film. The thickness of the GaN film depositedon the GaN substrate is 2 micrometers.

In order to use a GaN supporting body having the off-angle of a desiredvalue in the above experiment, a GaN substrate of 2-inch diameterfabricated in the above method is divided into a number of dedicated GaNsupporting bodies for experiments each having 10-millimeter square.Since the off-angle variation of the GaN substrate that the inventerscan be obtained is about 0.3 degrees, the off-angle variation of the GaNsupporting bodies is estimated to be within about 0.06 degrees. Thethickness variation of the GaN supporting bodies is within 1 micrometer.

A GaN film is deposited on each GaN supporting body. The Hallmeasurement of the deposited film shows as follows: the mobility is 200cm²V⁻¹seq⁻¹; the carrier concentration 5×10¹⁸ cm⁻³; the full-width halfmaximum by OMEGA scanning of (0002) face using X ray diffraction methodis 100 arcsec. Accordingly, this GaN film has an excellent crystalquality. In this measurement, a crystal analysis X ray diffractionapparatus for material of thin films, for example X′ Pert MRD system, isused.

The surfaces of the epitaxial films are analyzed using an Atomic ForceMicroscope apparatus. As shown in FIG. 4, the surface roughness becomeslarge as the off-angle is increased in the range of the off angle equalto or more than 0.5 degrees. GaN-based films, such as GaN film, grown onthe GaN supporting bodies are observed by use of a differentialinterference microscope and the observations are summarized as below.

As shown in FIG. 4, projections and depressions shaped in six-sidedpyramids are observed in the range OA1 of the off-angle equal to or lessthan 0.15 degrees. Step-like patterns are observed in the range OA2 of0.15 to 0.3 degrees. Planar surfaces are provided in the range OA3 of0.3 to 0.7 degrees. Scratch-like patterns are observed in the range OA4of 0.7 to 2 degrees. Deep scratch patterns are observed in the rangeequal to or more than 2 degrees.

In the single crystal GaN substrate according to the embodiment, theoff-angle distribution on the primary surface is adjusted. By depositinga Al_(X1)In_(Y1)Ga_(1-X1-Y1)N (0≦X1≦1, 0≦Y1≦1, 0≦X1+Y1≦1) film, such asGaN film, on the adjusted GaN substrate its surface morphology andsurface roughness (Rms) is becomes excellent as shown below.

For example, the surface morphology shows six-sided pyramid-like,step-like, atomic step-like, or scratch-like patterns and its surfaceroughness (Rms) is reduced to be equal to or less than 2 nanometers.Light emitting diodes fabricated on this substrate near uniformly emitlight. The primary surface of surface roughness less than 2 nanometerscan be favorable for a base region for a quantum well structure activeregion including a well layer of 1 to 5 nanometer thickness.

For example, in the off-angle range of 0.15 to 2 (exclusive) degrees(0.15≦off-angle <2), the surface morphology shows step-like, atomicstep-like and scratch-like patterns and the surface roughness (Rms) isreduced to be 2 nanometers at a maximum. Light emitting diodesfabricated on this substrate near uniformly emit light.

For example, in the off-angle range of zero (exclusive) to 0.7 degrees(0<off-angle ≦0.7), the surface morphology shows six-sided pyramid-like,step-like and atomic step-like patterns and the surface roughness (Rms)is reduced to be 0.5 nanometers at a maximum. Light emitting diodesfabricated on this substrate near uniformly emit light.

For example, in the off-angle range of 0.15 (exclusive) to 0.7 degrees(0.15<off-angle ≦0.7), the surface morphology shows step-like and atomicstep-like patterns and the surface roughness (Rms) is reduced to be 0.5nanometers at a maximum. Light emitting diodes fabricated on thissubstrate uniformly emit light.

For example, in the off-angle range of 0.3 (exclusive) to 0.7 degrees(0.3<off-angle ≦0.7), the surface morphology has shows flatnesscorresponding to atomic step-like patterns and the surface roughness(Rms) is reduced to be 0.3 nanometers at a maximum. Light emittingdiodes fabricated on this substrate uniformly emit light all over theemitting surface. For example, since the off-angle variation in a 2-inchdiameter GaN wafer is within 0.3 degrees, the excellent surfacemorphology is realized all over the surface of this GaN substrate.

FIG. 5 shows the off-angle distribution on the primary surface of theGaN single crystal substrate shown in part (B) of FIG. 3. The minimumand maximum values of the off-angle are located in a region between theedge and a line 3 millimeters apart from this edge. In this substrate13, the off-angle variation is in the range of 0.3 to 0.7 degrees. Inthe distribution shown in FIG. 5, stripe regions having the respectiveoff-angle ranges of 0.3 to 0.4 degrees, 0.4 to 0.5 degrees, 0.5 to 0.6degrees and 0.6 to 0.7 degrees are sequentially arranged from thebottom. When a III-group nitride film is grown on the present substrateof 2-inch diameter, the surface morphology of the deposited film isimproved over all the surface.

FIG. 6 shows the off-angle distribution on the primary surface of theGaN single crystal substrate shown in part (D) of FIG. 3. In theoff-angle distribution in FIG. 6, stripe regions having the respectiveoff-angle ranges of 0.2 to 0.4 degrees, 0.4 to 0.6 degrees, 0.6 to 0.8degrees, 0.8 to 1.0 degrees, 1.0 to 1.2 degrees, 1.2 to 1.4 degrees, 1.4to 1.6 degrees, 1.6 to 1.8 degrees and 1.8 to 2.0 degrees aresequentially arranged from the bottom up to the figure next to a striperegion having the off-angle range of 0 to 0.2 degrees. In this substrate15, the off-angle at a position near the edge is zero. The off-angle isincreased at a position as this position is distanced from the edge. Ata position on the other side of the edge, the off-angle is equal to orless than 2 degrees and is appropriate to 2 degrees.

FIG. 7 shows the off-angle distribution on the primary surface ofanother single crystal GaN substrate. In this substrate 17, the offangle does not take zero at any position all over the primary surface.In the distribution of FIG. 7, stripe regions having the respectiveoff-angle ranges of 0.2 to 0.25 degrees, 0.25 to 0.3 degrees, 0.3 to0.35 degrees, 0.35 to 0.4 degrees, 0.4 to 0.45 degrees, 0.45 to 0.5degrees, 0.5 to 0.55 degrees, 0.55 to 0.6 degrees, 0.6 to 0.65 degrees,0.65 to 0.7 degrees and 0.7 to 0.75 degrees are sequentially arrangedfrom the bottom to top of the figure next to a stripe region having theoff-angle range of 0.15 to 0.2 degrees. The minimum off-angle and themaximum off-angle are located in a range between the edge of thesubstrate and a line distanced from this edge by 3 millimeters. Theoff-angle minimum is 0.15 degrees and the off-angle maximum is 0.7degrees. When a GaN film is epitaxially grown on this substrate havingthe above off-angle variation, the surface morphology is step-like orplanar. The curvature radius of C-plane is 5.5 meters.

When off-angle Angle is small, the off angle is expressed in thefollowing relationship using curvature radius R and distance L from thereference position at which the off-angle is zero. FIG. 8 shows therelationship between the distance from the reference position and theoff-angle. In FIG. 8, lines L1 to L6 for curvature radius are shown, andthese lines correspond to the curvature radius R of C-plane. The linesL1 to L6 show the relationships for the curvature radius of 1 meter, 1.5meters, 2.0 meters, 2.5 meters, 3 meters and 5 meters. In the 2-inchdiameter substrate, if the curvature radius R of C-plane is equal to ormore than 1.5 meters, the off-angle variation is not more than 2degrees. Further, an ingot having the curvature radius R of C-plane of7.5 meters provides the off angle variation shown in FIG. 5.

As explained above, GaN ingots have convex or concave warpage. Whenplanar GaN substrate 11 is formed by providing slices from such a GaNingot with the processing of grinding and/or polishing, the off-angle isvaried over the primary surface. In the primary surface, linesindicating the same off-angles are drawn as concentric circles and/orconcentric circle arcs.

When a III-group nitride single crystal film is epitaxially grown on thesubstrate having a region which has the off-angle of zero or appropriatezero, the surface morphology of this film is not good on this region.Semiconductor devices on the region of better surface morphology haveexcellent characteristics as compared to those on the region of worsesurface morphology. If the off-angle of regions of the substrate onwhich semiconductor devices are formed is equal to or approximate zero,yield in the semiconductor device fabrication is lowered.

If the primary surface does not have a region of zero off-angle, thesurface morphology is improved and thus the yield of the semiconductordevice fabrication is enhanced.

On the contrary, the characteristics of semiconductor devices formed onthe outer region of the substrate are not excellent, for example, due toinhomogeneous gas flow in epitaxial growth. Thus, this outer region isnot used for the fabrication of semiconductor devices. Accordingly, itis preferable that the ingot be sliced to form a substrate(s) in which aposition of zero off-angle is located in the outer region.

Referring FIGS. 5, 6 and 7, lines indicate the same off-angles on theprimary surface of the single crystal GaN substrate. These linescorrespond to off-angle values more than zero, respectively. These linesterminate at the edge of the primary surface (the edge of thesubstrate), and are not closed on the surfaces 7 a, 11 a of thesubstrates 7, 11. These lines are concave or convex curves.

According to the GaN substrates 7 and 11, it is preferable that theoff-angle be equal to or more than 0.15 degrees over the second area ofthe primary surfaces 7 c, 11 c. The surface morphology of a filmdeposited on the substrate 7, 11 does not show six-sided pyramidpatterns. Further, it is preferable that the off-angle be equal to ormore than 0.15 degrees over the first and second area of the primarysurfaces 7 a, 11 a.

According to the GaN substrates 7 and 11, it is preferable that theoff-angle be equal to or more than 0.3 degrees over the second area ofthe primary surface 7 c, 11 c. The surface morphology of a filmdeposited on the substrate 7, 11 becomes substantially flat. Further, itis preferable that the off-angle be equal to or more than 0.3 degreesover the first and second area of the primary surface 7 a, 11 a.

In the GaN substrate 7, 11, it is preferable that the off-angle be equalto or more than 2 degrees over the second area of the primary surfaces 7c, 11 c. The surface morphology of a film deposited on the substrate 7,11 does not show scratch-like roughness.

In the GaN substrate 7, 11, it is preferable that the off-angle be equalto or more than 0.7 degrees over the second area 7 c, 11 c. The surfacemorphology of a film deposited on the substrate 7, 11 has six-sidedpyramid patterns, step-like patterns, or atomic step-like patterns, andthe surface roughness (Rms) is educed to be 0.5 nanometers at a maximum.Further, it is preferable that the off-angle be equal to or more than0.7 degrees over the first and second area 7 b, 11 b, 7 c, 11 c of theprimary surfaces 7 a, 11 a.

As explained above, the present embodiment can provide the GaN substratefor yielding more nitride semiconductor devices, and the method offabricating the GaN substrate.

Parts (A) to (C) in FIG. 9, parts (A) and (B) in FIG. 10 show the methodof forming a nitride semiconductor epitaxial substrate and the method offabricating GaN-based semiconductor device according to the embodiment.

A single crystal GaN substrate 21 is prepared. The single crystal GaNsubstrate 21 can be fabricated by the method according to the firstembodiment. Prior to the deposition of a III-group nitride film, thepreprocessing of the single crystal GaN substrate 21 is performed in theOMVPE apparatus 23. The single crystal GaN substrate 21 is provided onthe suceptor 25 in the OMVPE apparatus 23. The single crystal GaNsubstrate 21 is thermally processed while flowing process gas to flattenthe surface of the substrate 21. This thermal process onto the surface21 a of the substrate 21 reduces scratches caused by mechanicalpolishing. In the preferable example, the process gas contains NH₃ andH₂.

As shown in part (B) in FIG. 9, a GaN-Based film 29 is formed on theprimary surface 21 a of the substrate 21 while supplying raw materialgas. In the present example, a GaN film is deposited directly on theprimary surface 21 a of the substrate 21 using the OMVPE apparatus 23.The temperature of the substrate is set at a temperature higher than thetemperature in the preprocessing step. The raw material gas containstrimethyl-gallium (TMG), ammono (NH₃), hydrogen (H₂) and nitrogen (N₂).If required, silane (SiH₄) gas can be used as n-type dopant. After thisstep, a nitride semiconductor epitaxial substrate 32 including aIII-group nitride film formed on the GaN substrate.

As shown in part (C) in FIG. 9, a device region is formed. One or moreanother III-group nitride films 31, such as an active layer, for thedevice region are formed on the primary surface 21 a. The active layercan have a single quantum well structure (SQW) or a multiple quantumwell structure (MQW). In the present example, the active layer of theMQW structure is formed, and the temperature of the substrate is loweredas compared to the substrate temperature for the GaN film growth. Forexample, an InGaN film is grown for a well layer of the MQW structure,and another InGaN film of the bandgap greater than that of the welllayer is grown for a barrier layer of the MQW structure. For example,the MQW structure of five well layers is formed for a light emittingdiode. In this step, an epitaxial substrate including a number ofIII-group nitride films has formed on the GaN substrate is obtained.

Next, as shown in part (C) in FIG. 9, a still another III-group nitridefilm 33 is formed on the primary surface 21 a of the GaN substrate 21.In the present example, an AlGaN film is grown on the active layer usingthe OMVPE apparatus 23. The substrate temperature is higher than thegrowth temperature for the active layer. A raw material gas for thisgrowth can contain trimethyl-gallium (TMG), trimethyl-aluminum (TMAl),ammono (NH₃), hydrogen (H₂) and nitrogen (N₂). As required,cyclo-penta-dienyl-magunesium (CP₂MG) is used as p-type dopant. Forexample, a Mg-doped AlGaN film are grown to obtain p-type AlGaN film.After this step, a nitride semiconductor epitaxial substrate including anumber of III-group nitride films has formed on the GaN substrate.

As shown in part (C) in FIG. 9, another III-group nitride-based film 35is formed on the primary surface 21 a of the GaN substrate 21. In thepresent example, a Mg-doped GaN film is grown on the p-type AlGaN filmto form the p-type GaN film. In this step, a nitride semiconductorepitaxial substrate 37 including a number of III-group nitride filmsformed on the GaN substrate.

As shown in part (C) in FIG. 9, an n-type ohmic electrode 39 is formedon the backside 21 b of the GaN substrate. A p-type ohmic electrode 41and a pad electrode 43 are formed on the epitaxial film formed in thestep in part (C) in FIG. 9.

After the formation of the pad electrode 43, the substrate is cut alongthe broken lines CUT1 and CUT2 to form semiconductor light emittingdevices 51, such as light emitting diode. By use of the above steps, aIII-group nitride semiconductor device is manufactured.

As explained above, the present embodiment provides the method offabricating a nitride-based semiconductor device. Further, the presentembodiment also provides the method of fabricating a nitridesemiconductor epitaxial substrate. This nitride semiconductor epitaxialsubstrate includes a single crystal GaN substrate having a primarysurface of the predetermined off-angle variation, one or more III-groupnitride semiconductor layers formed on the single crystal GaN substrate.The nitride semiconductor substrate provides these III-group nitridesemiconductor layers with excellent surface morphology.

The following epitaxial substrate can be fabricated using the method ofmaking a nitride semiconductor epitaxial substrate according to thepresent embodiment. This nitride semiconductor epitaxial substratecomprises a single crystal GaN substrate having a primary surface of thepredetermined off-angle variation, a first conductive typeAl_(X1)In_(Y1)Ga_(1-X1-Y1)N (0≦X1≦1, 0≦Y1≦1, 0≦X1+Y1≦1) film formed onthe GaN substrate, an active layer including anAl_(X2)In_(Y2)Ga_(1-X)2-Y2N (0≦X2≦1, 0≦Y2≦1, 0≦X2+Y2≦1) film formed onthe GaN substrate, and a second conductive typeAl_(X3)In_(Y3)Ga_(1-X3-Y3)N (0≦X3≦1, 0≦Y3≦1, 0≦X3+Y3≦1) film formed onthe GaN substrate. The active layer is provided between the firstconductive type Al_(X1)In_(Y1)Ga_(1-X1-Y1)N film and the secondconductive type Al_(X3)In_(Y3)Ga_(1-X3-Y3)N film. According to theepitaxial substrate, the first conductive typeAl_(X1)In_(Y1)Ga_(1-X1-Y1)N (0≦X1≦1, 0≦Y1≦1, 0≦X1+Y1≦1) film, theAl_(X2)In_(Y2)Ga_(1-X2-Y2)N (0≦X2≦1, 0≦Y2≦1, 0≦X2+Y2≦1) film and thesecond conductive type Al_(X3)In_(Y3)Ga_(1-X3-Y3)N (0≦X3≦1, 0≦Y3≦1,0≦X3+Y3≦1) film have excellent surface morphology.

In one example, the epitaxial substrate includes:

the thickness of n-type GaN substrate: 400 micrometers;the thickness of n-type GaN film: 1 micrometer;undoped In_(0.15)Ga_(0.85)N well layer: 2 nanometers;undoped In_(0.01)Ga_(0.99)N well layer: 15 nanometers;p-type AlGaN layer: 20 nanometers;p-type GaN layer: 50 nanometers.

Part (A) in FIG. 11 shows the distribution of photoluminescencewavelength of the epitaxial substrate that uses the substrate formed bythe method in part (A) in FIG. 2. Part (B) in FIG. 11 shows thehistogram of the wavelength distribution of part (A) in FIG. 11. Thevariation of the off-angle on the primary surface is in the range of0.15 to 0.7 degrees. Part (A) in FIG. 12 shows the distribution ofphotoluminescence intensity of the epitaxial substrate that uses thesubstrate formed by the method in part (A) in FIG. 2. Part (B) in FIG.11 shows the histogram of the intensity distribution of part (A) in FIG.11. Parts (A) of FIGS. 11 and 12 show the measurement for epitaxialsubstrates fabricated by use of 2-inch GaN substrates.

Parts (A) and (B) in FIG. 11 reveal average of 441.5 nanometers,standard deviation of 1.65 nanometers and distribution width of −2nanometers to +2 nanometers.

According to the measurement in parts (A) and (B) of FIG. 12, value(SIG/AVE) derived by dividing the standard deviation SIG of theintensity by the average AVE of the intensity is about 15%. This showsthe emitting is substantially uniform over the surface. On the contrary,in the substrate having off-angle variation in the range of zero to 0.15degrees (exclusive) and more than 2 degrees, the photoluminescenceintensity is lowered and the photoluminescence wavelength is shifted toa longer wavelength region, which lower the semiconductor device yield.

Having described and illustrated the principle of the invention in apreferred embodiment thereof, it is appreciated by those having skill inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. Details of structures of thesedevices can be modified as necessary. For example, althoughsemiconductor light-emitting diodes are described in the embodiment, thepresent invention is not limited to the specific examples disclosed inthe embodiments. Further, a GaN film formed on the GaN substrate isexplained in the embodiment, but III-group nitride semiconductor(Al_(X)In_(Y)Ga_(1-X-Y)N, 0≦X≦1, 0≦Y≦1, 0≦X+Y≦1) can be formed. Wetherefore claim all modifications and variations coming within thespirit and scope of the following claims.

1. A method of fabricating a single crystal gallium nitride substrate,the method comprising the step of: cutting an ingot of single crystalgallium nitride along predetermined planes to make one or more singlecrystal gallium nitride substrates, the ingot of single crystal galliumnitride being grown by vapor phase epitaxy in a direction of apredetermined axis, each predetermined plane being inclined to thepredetermined axis, each substrate having a mirror polished primarysurface, the primary surface having a first area and a second area, thefirst area being between an edge of the substrate and a line 3millimeter away from the edge, the first area surrounding the secondarea, an axis perpendicular to the primary surface forming an off-anglewith c-axis of the substrate, the off-angle taking a minimum value at afirst position in the first area of the primary surface.
 2. The methodaccording to claim 1, wherein an axis perpendicular to the primarysurface forms an angle greater than zero with c-axis of the substrate.3. The method according to claim 1, wherein the off angle takes amaximum value at a second position on the primary surface, and the offangle makes substantially monotonic change on a segment from the firstposition to the second position.
 4. The method according to claim 1,wherein the off angle takes a nonzero constant value on a curve on theprimary surface and the curve terminates at the edge of the primarysurface.
 5. A method of fabricating a single crystal gallium nitridesubstrate, the method comprising the step of: slicing an ingot of singlecrystal gallium nitride along predetermined planes to prepare one ormore single crystal gallium nitride slices, each predetermined planebeing inclined to a predetermined axis, the ingot of single crystalgallium nitride being grown by vapor phase epitaxy, and each singlecrystal gallium nitride slice having a slice surface; and performing atleast one process of polishing and grinding of the slice surface to formone or more single crystal gallium nitride substrates, each substratehaving a primary surface, the primary surface having a first area and asecond area, the first area being between an edge of the substrate and aline 3 millimeters away from the edge, the first area surrounding thesecond area, an axis perpendicular to the primary surface forming anoff-angle with c-axis of the substrate, the off-angle taking a minimumvalue at a first position in the first area of the primary surface. 6.The method according to claim 5, wherein an axis perpendicular to theprimary surface forms an angle greater than zero with c-axis of thesubstrate.
 7. The method according to claim 5, wherein the off angletakes a maximum value at a second position on the primary surface, andthe off angle makes substantially monotonic change on a segment from thefirst position to the second position.
 8. The method according to claim5, wherein the off angle takes a nonzero constant value on a curve onthe primary surface and the curve terminates at the edge of the primarysurface. 9-28. (canceled)